In many instances, damage to the spine as a result of advancing age, disease and injury is treated by fixation or stabilization of vertebrae. Conventional methods of spinal fixation utilize spinal fusion, a procedure in which bone growth is encouraged to bridge the disc space to fuse together adjacent vertebrae and as a result, stabilize the spinal motion segment. Spinal fusion involves at least partial removal of a damaged intervertebral disc and the introduction of bone graft material that is typically contained in an interbody spacer which is implanted into the intervertebral disc space. Kidney bean-shaped, curved or other shaped interbody spacers or cages made of titanium or polyether ether ketone (PEEK) are employed to provide decompression and house the bone graft material. The bone graft material is usually supplemented with bone morphogenic protein, demineralized bone matrix in the form of paste or cement, stem cells or other oseoinductive biological agents which are known to enhance fusion.
There are many different types of spinal fusion procedures. One is anterior lumbar interbody fusion or “ALIF”. In ALIF, the patient is placed on their back and the section of the spine to be fused is approached through the front of the patient through incisions in the abdomen. The abdominal organs are moved aside, the damaged disc space is exposed and bone graft material encaged in an interbody spacer is implanted. Another spinal fusion technique is posterior lumbar interbody fusion (“PLIF”) in which the spine is approached and bone graft material is implanted through the back of a patient lying prone on a surgical table. A type of PLIF is transforaminal lumbar interbody fusion or “TLIF”. TLIF is another widely used method of spinal fusion for the treatment of a variety of lumbar spinal disorders when avoidance of complex anterior approaches and diminished posterior trauma is desired. In TLIF, the spine is approached through the backside of the patient as in PLIF; however, access to the interbody space is gained through the foramina. Another surgical spinal fusion technique is extreme lateral interbody fusion, also known as XLIF, which is performed through the patient's side, avoiding the major muscles of the back. Other surgical spinal fusion techniques include direct lumbar interbody fusion (“DLIF”), lateral trans-psoas interbody fusion (“SLIF”) and intertransverse lumbar interbody fusion (“ILIF”).
To lower the rate of imperfect fusion, i.e. pseudoarthrosis, adjuncts to spinal fusion have been employed. Most commonly, posterior instrumentation such as pedicle screws and rods are implanted on the posterior side of the spine. Four pedicle screws are typically placed into the pedicles with two pedicle screws in each adjacent vertebra. Two rods interconnect the screws of the adjacent vertebra and span the intervertebral disc space at issue. The rods are typically secured to the head of each screw with caps that are tightened down to capture the rod between the cap and screw. Other posterior instrumentation such as plates or other interconnectors may also be employed and secured to screws implanted into the pedicles.
Generally, pedicle screw fixation requires a high degree of skill that involves locating the pedicle that is not always directly visualized or may be difficult to visualize due to aberrant anatomy. Further skill is required to deliver the pedicle screw in the appropriate location with the proper angulation and the correct trajectory. Even in straightforward anatomical situations, pedicles can have various diameters and significant variations in trajectory are possible. For these reasons, fluoroscopic imaging is typically used by many surgeons to facilitate the surgical application of pedicle screws which increases the radiation exposure to the patient and surgical team. Other complications of pedicle screw placement include duration of the procedure, significant tissue dissection and muscle retraction, scarring, misplaced screws, excess rigidity leading to adjacent level disease requiring further fusions and re-operations.
An alternative to pedicle screw fixation is facet fixation in which the facet joint is significantly immobilized. Each vertebra has a pair of articular surfaces located on the left side and a pair of articular surfaces located on the right side. Each pair includes a superior articular surface that forms a facet joint with the adjacent higher vertebra and an inferior articular surface that forms a facet joint with the adjacent lower vertebra. Together the superior and inferior articular surfaces of adjacent vertebrae form a facet joint. Facet joints are synovial joints that are surrounded by a capsule of connective tissue with synovial fluid nourishing and lubricating the joint. The joint surfaces are coated with cartilage allowing the joints to articulate relative to one another. With adjacent intervertebral bodies fused together in a spinal fusion procedure, the fixation of the facet provides an additional supplement to spinal fusion. Facet fixation can also be employed as a stand alone procedure or primary means to stabilize the spine without fusion of intervertebral bodies. Facet fixation can also serve to augment pedicle screw fixation or other posterior instrumentation on one side of the spine.
Typically, in facet fixation, a single facet screw is inserted directly across the facet joint to fix or limit the motion of the facet joint. In one variation of facet screw fixation called translaminar facet screw fixation, screws are inserted from the base of the spinous process on the contralateral side and through the lamina to traverse the facet joint indirectly and into the pedicle of the successively inferior vertebra. In translaminar facet screw fixation, the translaminar facet screws are longer than the screws used in direct facet fixation. To fixate both facet joints of a motion segment, two translaminar facet screws are placed in crisscross fashion across the lamina. Other procedures involve positioning an implant in the facet joint between the articular faces which may require removal of bone and the implantation of bone graft and/or growth material with or without a facet screw across the joint. Where the lamina is weak, translaminar facet screws can toggle and even break the lamina and destabilize the joint. However, facet screw fixation offers significant advantages when compared to pedicle screw fixation. For example, bilateral facet fixation uses only two screws per level replacing four screws, two rods and associated caps used in pedicle screw fixation. Facet fixation requires less hardware, less operating time and is easier to implant. Furthermore, facet screw strength and stability is comparable to traditional pedicle screws. Facet screws also offer the potential reduction in fluoroscopic imaging resulting in less exposure to radiation due to less hardware to implant. Furthermore, facet fixation preserves adjacent level anatomy compared to pedicle screw fixation. Because of these and other advantages offered by facet screws, they are now being implanted on a regular basis. In order to further improve upon the use of such facet screws, an improved facet screw and a minimally invasive method for accurately and repeatedly placing facet screws across the facet joints is needed. This invention provides an improved facet screw and novel method of implantation.